Device-grade thin-film CuInSe2 was subjected to various chemical treatments commonly used in photovoltaic device fabrication to determine the resulting microscopic surface composition/morphology and the effect on II-VI/CuInSe2 heterojunction formation. HCI (38%), Br-MeOH (<1% Br), (NH4)2S, and NH4OH/thiourea solutions were used separately to modify the surface chemistry of the CuInSe2 polycrystalline films. Scanning electron microscopy was used to evaluate the resultant surface morphology. Angle-resolved high-resolution photoemission measurements on the valence band electronic structure and Cu 2p, In 3d, Ga 2p and Se 3d core lines were used to evaluate the chemistry of the chemically treated surfaces. CdS overlayers were then deposited in steps on these chemically treated surfaces. Photoemission measurements were acquired after each growth to determine the resultant heterojunction valence-band discontinuity between the CdS and the chemically modified CuInSe2 surface.

CuInS2 films were grown by molecular beam epitaxy (MBE) on hydrogen terminated Si(111) substrates with 4° miscut. X-ray diffraction (XRD) texture analysis reveals that CuInS2 was grown heteroepitaxially with the epitaxial relationships CuInS2(112) II Si(111) and [111] II [111]. Moreover, a substantial amount of rotational twins is observed. The crystalline order is maintained across the interface as observed by cross-sectional transmission electron microscopy (XTEM). XRD and scanning electron microscopy (SEM) investigations show that nonstoichiometric preparation greatly influences the growth morphology and leads to the formation of secondary phases.

The conductivity σ of CuInS2 films grown by coevaporation is investigated by in-situ measurements during the deposition and ex-situ measurements. During the controlled cool-down period of the grown films, σ vs l/T data are obtained for films with Cu-rich and In-rich composition. Furthermore, first experiments on the effect of extrinsic dopants such as P, N, and Na on the conductivity of In-rich films are reported.

CdTe/CdS solar cells have demonstrated stability over thousands of hours of field testing, but have not yet established the same track record for reliability achieved by crystalline Si PV modules. In order to identify potential degradation mechanisms, this paper describes a series of experiments in which CdTe/CdS solar cells are subjected to combinations of temperature, voltage and illumination levels intended to stress the devices beyond levels which would be experienced in the field. During this process the devices display decreases in Voc and FF which are tentatively attributed to chemical changes at the low resistance contact to the CdTe and to changes in the electrical transport properties of the CdTe. Two thermally activated effects were observed depending upon stress bias. Reverse bias appears to degrade bulk CdTe electrical properties while forward bias strengthens a reverse diode which most likely is located at the back contact. Chemical analysis of the back contact region of the devices using XPS reveals strong oxide peaks in all stressed samples. Evidence of Cd diffusion into the Mo contacts is observed by SIMS and Auger. Mo diffusion into the CdTe is promoted by reverse bias stressing. Cu migration consistent with positively charged ions was indicated by SIMS.

Thin films of CdTel-xSx with bulk atomic compositions, x≡[S]/([S]+[Te]), ranging from 0 to 0.45 were deposited by vacuum co-evaporation of CdTe and CdS with substrate temperatures of 200 and 250°C. X-ray diffraction analysis revealed that films with x < 0.3 were predominately single phase having the zincblende structure. Films with 0.35 < x < 0.45 contained the wurtzite modification. Lattice parameter determination indicated that each phase exists with compositions well within the miscibility gap shown on published equilibrium phase diagrams. The variation of the optical band gap with x was determined by measuring transmission and reflection of the films. Heat treatment at 415°C in the presence of CdC12 caused the films to segregate into two phases consistent with the phase diagram. If the CdCl2 is assumed to only promote the phase segregation process, then the compositions of the two phases after heat treatment may be taken as measurements of the solubility limits of S in CdTe and Te in CdS respectively. The solubility limit of S in CdTe was thus determined to be 5.8% at 415°C which is the temperature used for the common CdC12 treatment of CdTe-based solar cells. An analysis of CdTe/CdS solar cell device structures shows that the atomic composition of alloys created by interdiffusion are consistent with these solubility limits.

We investigated the effect of CdCl2 treatment of CdS films on the photovoltaic performance of polycrystalline CdTe/CdS solar cells. X-ray diffraction studies indicated that the diffusion of S into CdTe is qualitatively the same for CdTe/CdS films fabricated with both as-deposited and CdCl2-treated CdS. A major difference was observed in the extent of Te diffusion into CdS for the two types of CdS films. Full conversion of CdS into CdS1-yTey; was observed for films prepared with asdeposited CdS, while the formation of the ternary phase was below the detection limit for films prepared with CdCl2-treated CdS. Photoluminescence measurements confirmed this result. The difference in interdiffusion leads to differences in optical transmission of CdS films and spectral response of CdTe/CdS solar cells. An increase of 2.7 mA/cm2 in short-circuit current density was observed as a result of improved spectral response in the wavelength range of 500–600 nm for the CdCl2-treated CdS.

The presence of oxygen during close-spaced sublimation (CSS) deposition of CdTe has been regarded as necessary for high-efficiency CdS/CdTe solar cells. To date, research has focused on the effect of oxygen on the acceptor density in the CdTe absorber. We find that although oxygen may influence the doping density, a perhaps more significant effect is its influence on nucleation and growth of the film. Oxygen partial pressures of 0 to 24 torr and total pressures of 5 to 45 torr (balance He) have been explored for CSS depositions. We find that increasing the O2 partial pressure increases the density of CdTe nucleation sites, thereby suppressing pinhole formation and likely reducing defects at the junction. However, increasing O2 also tends to decrease grain size and faceting in the films. In addition to influencing the film properties, O2 adversely affects the source material, producing non-uniformly oxidized surfaces that reduce deposition rates and impair device uniformity and run-to-run reproducibility.

We have also determined that the graphite susceptors typically used with CSS deposition convert a portion of the oxygen ambient to CO and CO2 during deposition. The amount converted is highly dependent on the type and age of the graphite susceptor used. The COx byproducts are not efficiently incorporated in the CdTe film and do not influence the deposition as oxygen does. We describe the effect of the changing oxygen partial pressure during the course of deposition. Finally, we report high-efficiency solar cells (12.8% AM 1.5) made without oxygen.

CdTe thin film growth using nanoparticle precursors and spray deposition has been investigated. Employing a metathesis approach, cadmium iodide was reacted with sodium telluride in methanol solvent resulting in the formation of soluble Nal and insoluble CdTe nanoparticles. After appropriate chemical workup, methanol-capped CdTe colloids were isolated. CdTe colloids prepared by this method exhibit a dependence of the nanoparticle diameter upon reaction temperature as determined by transmission electron microscopy (TEM) and UV-Visible spectroscopy (UV-Vis). CdTe thin film formation was achieved by spray depositing the nanoparticle colloids (25–75 Å diameter) onto substrates at elevated temperatures (T = 280–440 °C) with no further thermal treatment. These films were characterized by XRD, x-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). Cubic CdTe phase formation was observed by XRD with a contaminant oxide phase also detected. XPS analysis showed that CdTe films produced by this one-step method contained no Na or C, but substantial O. AFM gave CdTe grain sizes of ˜0.1–0.3 pim for films sprayed at 400 °C. A layer-by-layer film growth mechanism proposed for the one-step spray deposition of nanoparticle precursors will be discussed.

Vacuum-evaporated Cu-doped ZnTe films have been studied as the intermediate layer between CdTe and metal contacts in CdTe/CdS thin-film solar cells for the formation of low resistance back contacts. Different metals (Au, Ni, Co) have been investigated as the contact material to the ZnTe layer. The effects of Cu concentration, ZnTe:Cu layer thickness, and ZnTe post-deposition annealing temperature on the cell performances have been investigated. We found that different metal contacts on the ZnTe layer lead to different doping densities in the CdTe layer and different open-circuit photovoltages of the solar cells. The possible formation of a back contact diode at the CdTe/ZnTe interface was explored, based on capacitance-voltage analysis. The series resistance of the CdTe/CdS cells was reduced significantly by the introduction of the ZnTe layer. Fill factors greater than 0.76 and an energy conversion efficiency of 12.9% have been achieved using ZnTe back contacts on electrodeposited CdTe.

A study of the CdS/CdTe interface was performed on glass/SnO2/CdS/CdTe device structures. CdS layers were deposited by chemical solution growth to a thickness of 80–100 nm, and CdTe was deposited by close-spaced sublimation at substrate temperatures of 500°, 550°, and 600°C. Post-deposition CdCl2 heat treatment was performed at 400°C. Samples were analyzed by optical spectroscopy, secondary ion mass spectrometry (SIMS), spectral response, and current-voltage measurements. SIMS analysis shows that the intermixing of CdS and CdTe is a function of substrate temperature and post-deposition CdCl2 heat treatment. The degree of intermixing increases with increases in substrate temperature and the intensity of CdCl2 heat treatment. Optical analysis and X-Ray diffraction data show that the phases of CdSxTe1-x are also a function of the same parameters. Formation of a Te-rich CdSxTe1-x alloy is favored for films deposited at higher substrate temperatures. Spectral response of the devices is affected by the degree of alloying at the interface. The degree of alloying is indicated by simultaneous changes in long wavelength response (due to the formation of lower bandgap intermixed CdSxTe1-x) and the short wavelength response (due to the change in CdS thickness). Device performance is heavily influenced by alloying at the interface. With optimized intermixing, improvements in Voc, and diode quality factors are observed in the resulting devices.

Interdiffusion of sulfur and tellurium across the CdS/CdTe interface is fundamentally important in the operation of CdTe solar cells. However, the properties of the resulting alloy semiconductor, CdSxTe1-x, are not well understood. We have prepared films of this ternary material by pulsed excimer laser deposition (PLD) across the alloy range. These films were examined by x-ray diffraction, wavelength dispersive x-ray spectroscopy, optical absorption, and Raman scattering to determine the influence of sulfur content on the crystal structure, lattice constant, energy gap, and vibrational mode behavior.

Samples of the (CdTe)1-xTex diphasic system were grown by HWFE on glass plates kept at temperatures between 150 and 260 °C. The films have excess elemental tellurium varying between 12 and 67 vol%. X-ray analyses showed the samples to be diphasic with CdTe in the cubic phase and Te in the hexagonal phase. The excess tellurium concentration increased as the substrate temperature diminished. The critical volume fraction for conductivity percolation in the (CdTe)1-xTex system was determined to be around 0.4, which indicates a non-random distribution of the excess Te. Raman spectroscopy was used to characterize the Te phase across the conductivity percolation threshold. The integrated intensity of the first-order A1 mode increases dramatically at the percolation transition. Second-order Raman scattering from Te is also seen, but only for those samples with over 38 vol% Te. In addition, the A1 mode is shifted to higher energies in films with concentrations below the transition point, revealing that Te goes from a state of tensile stress into a relaxed state as the films pass from low to high conductivity. These results indicate that simultaneously with the Te network connectivity there occurs aggregation of the excess Te, which was initially located at the grain surfaces. This mechanism relieves the strain and gives rise to high conductivity and extended vibrational modes resembling those of bulk elemental tellurium at volume concentrations around 40% and above.

Rapid thermal annealing (RTA) was applied to anneal polycrystalline CdTe thin films evaporated on CdS/ITO substrate and the effects of rapid thermal annealing temperatures and gas environments were studied. X-ray diffractometer (XRD), X-ray photoelectron spectroscopy(XPS), energy dispersive X-ray spectroscopy(EDX), cross-sectional transmission microscopy(TEM), and micro-EDX in TEM were used to characterize physical and chemical properties of rapid thermal annealed CdTe thin films. Complete CdTe/CdS photovoltaic cells were fabricated and I-V characteristics of these cells were measured under the illumination. Results showed that the bulk composition of CdTe remained stoichiometric to 550°C in the air environment and surface composition became Cd-rich. Cross-sectional TEM and micro-EDX showed columnar grains and micro-twins remained even after RTA, however, sulfur content in rapid thermal annealed CdTe caused by sulfur diffusion from CdS during the annealing was much smaller than that by furnace annealing. Among the investigated RTA temperatures and gas environments, the cell made with CdTe annealed at 550°C in the air showed the best solar energy conversion efficiency.

CdTe films have been prepared by a close-spaced sublimation (CSS) process with screenprinted CdTe layers as a source. The electrical resistivity of the CdTe film deposited in O2 was about one order of magnitude lower than that of the film deposited in He. The Cd content in the CdTe films deposited in O2 was smaller than that in the CdTe films deposited in He. No oxygen was detected in the CdTe films deposited in O2. As the Cd/Te ratio decreased, the resistivity of the CdTe films decreased and reached at about 3×104 Ω2.cm. The resistivity decrease by oxygen flow was diminished when the CdTe composition was limited by solid solubility. The above results indicated that the resistivity decrease of CdTe films deposited in O2 was not by due to the oxygen doping but by due to the CdTe stoichiometry change.

We report studies of solar cells prepared by rf planar magnetron sputtering in which the films were deposited using magnetic field structures ranging from approximately balanced to strongly unbalanced in the “type II” configuration. For films grown with the unbalanced configurations, we find much stronger photoluminescence and much better cell performance than for the balanced configuration. The CdTe films show differences in electrical performance depending on magnetic field as well. These effects are interpreted as arising from the enhanced electron and ion bombardment of the film growth interface for the unbalanced magnetrons. Using two unbalanced magnetrons we have fabricated an all-rf-sputtered cell with NREL-verified efficiency of 11.66% at air mass 1.5 illumination.

The flat glass industry successfully commercialized pyrolytically deposited low emissivity glass based on color-suppressed fluorine-doped tin oxide in 1990. The current production capability of this product approaches 500,000 m2 of coated glass per year. Spin-off products for the photovoltaic (PV) industry were introduced by LOF in 1991 and enjoy considerable success in this emerging technology. Many PV researchers, however, still prefer to produce optimized tin oxide off-line in CVD furnaces as they can be produced on higher performing but higher cost substrates. In this paper, we discuss the characteristics of on-line production of PV transparent conductors (TCO), including the tradeoffs required when a coating process is incorporated onto a float glass line. We conclude by suggesting that eventual large scale commercialization of thin film photovoltaics dictates the use of on-line TCO based on economics, the excellent uniformity and repeatability, and ready availability. We believe that the PV researchers are best served by understanding the capabilities of on-line tin oxide production, and tailoring their programs towards the characteristics of these new cost effective TCOs.

A comparative study of several properties of ZnO:Al and ZnO:B films grown by magnetron sputtering has been performed. The influence of these window layers on device performance has also been investigated. Sputter-deposited ZnO:B films were used for the first time as a window material of CIS thin film solar cells. The short circuit current was improved by replacing ZnO:Al with ZnO:B window layers. A discussion of the issues relating to ZnO window layers for improving cell performance is presented.

Transparent, electrically conductive films have been prepared from several different metal oxides, including those of tin, indium and zinc. Deposition methods for these materials are reviewed, and their properties summarized and compared. A figure of merit for a transparent conductor may be defined as the ratio of the electrical conductivity to the optical absorption coefficient of the film. The figure of merit for fluorine-doped zinc oxide is shown to be larger than that of other transparent conductors, such as boron-doped zinc oxide, fluorine-doped tin oxide, and tin-doped indium oxide. Physical, chemical and thermal durability, deposition temperature, and cost are other factors which may also influence the choice of material for a particular application.

ITO films have been deposited onto glass substrates by rf reactive magnetron sputtering using In-Sn (90–10) alloy target. After the deposition, the films were annealed in air at 500 °C for 30, 60, 90 and 180 min respectively. The film structure varies as the annealing time is changed. The film electrical properties show a strongly dependence on the film structure. Although all the films show a preferred orientation along the (400) direction, the film which has high (222) diffraction peak intensity, has high carrier mobility and low resistivity.

The current-voltage (I-V) characteristics of the interfaces between boron-doped hydrogenated amorphous silicon (p-a-Si:H) and transparent conductive oxides (TCOs) such as tin oxide (SnO2) and indium tin oxide (ITO) are experimentally determined. The measurements are performed on especially developed Kelvin cross bridge structures which allow the direct sensing of the contact region. All contacts exhibit for high dopant concentrations an almost linear I-V characteristic. The values of the contact resistance depend on the TCO type and range between 0.5 and 4 Ω· cm2 at 25°C. Gradual reduction of the boron doping level leads to a remarkable increase of the contact resistance and finally to a rectifying behavior. The results indicate that tunnelling across the interface contributes significantly to the current transport. This conclusion is also supported by the behavior of the I-V characteristics at low temperatures. Regarding degradation of the contacts a systematic reduction of the contact resistance is observed after ageing them for 1000 hours at 80°C.